The answer is yes. OS X looks only for plist files when loading LaunchAgents or LaunchDaemons, regardless of whether you use the old style "Mach" way (which as bmike has pointed out has long been deprecated) or the current, modern method.
The contents of the plist file are as follows (this shows the AELWriter service):
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"> <plist version="1.0"> <dict> <key>Label</key> <string>com.apple.aelwriter</string> <key>MachServices</key> <dict> <key>com.apple.aelwriter</key> <true/> </dict> <key>ProgramArguments</key> <array> <string>/usr/sbin/AELWriter</string> </array> <key>RunAtLoad</key> <false/> <key>UserName</key> <string>root</string> <key>ThrottleInterval</key> <integer>2</integer> </dict> </plist>
You can see that what actually launches the service is a string under the key
ProgramArguments. Changing that value would also "disable" the service from running, as would removing the plist, renaming the file extension, or deleting the file outright.